Non-destructive inspection (NDI) of structures involves thoroughly examining a structure without harming the structure or requiring its significant disassembly. Non-destructive inspection is typically preferred to avoid the schedule, labor, and costs associated with removal of a part for inspection, as well as avoidance of the potential for damaging the structure. Non-destructive inspection is advantageous for many applications in which a thorough inspection of the exterior and/or interior of a structure is required. For example, non-destructive inspection is commonly used in the aircraft industry to inspect aircraft structures for any type of internal or external damage to or defects (flaws) in the structure. Inspection may be performed during manufacturing or after the completed structure has been put into service, including field testing, to validate the continued integrity and fitness of the structure.
During NDI, one or more sensors may move over the portion of the structure to be examined, and receive data regarding the structure. Various types of sensors may be used to perform non-destructive inspection. For example, a pulse-echo (PE), through transmission (TT), or shear wave sensor may be used to obtain ultrasonic data, such as for thickness gauging, detection of laminar defects and porosity, and/or crack detection in the structure.
In some circumstances, only a single surface of the structure may be accessible for inspection purposes, thereby limiting the potential inspection techniques. For example, in the field, access to interior surfaces of the structure is often restricted, requiring disassembly of the structure and introducing additional time and labor. Similarly, during manufacture, one of the surfaces may be disposed upon a mandrel and be inaccessible, at least without undesirable and time-consuming disassembly.
While single-sided inspection techniques, such as PE, can be employed to detect disbonds, delaminations, cracks or other substantial defects, it may be difficult to detect porosity in certain situations, such as situations in which the structure under inspection is ultrasonically coupled to another structure, such as a mandrel or other backing material, absent a TT inspection technique. In this regard, in PE, the amplitude of the reflection from the back surface, i.e., the surface opposite the inspection sensor, is used as a gage to determine the percent of porosity by comparing the reflection from the back surface of the structure under inspection with standard data gathered from prior inspections of reference samples of known porosity. Accordingly, porosity may be difficult to detect and/or quantify, especially in conjunction with structures that are only amenable of single-sided inspection and are ultrasonically coupled to another structure, since the ultrasonic coupling will reduce the reflection from the back surface by an unknown amount. Such difficulties in accurately detecting and/or quantifying porosity may be problematic in composite manufacturing processes in which it is desirable to monitor the quality of the composite material including, for example, the porosity of the composite material to insure that the manufacturing process is performing in the desired manner.
While it is generally desirable to detect porosity during or following manufacture, it is similarly desirable to be able to identify microcracking or thermal damage in the field or otherwise once the composite material has been placed in service. Microcracking can occur due to fatique or thermal cycling of composites. Microcracks generally consist of multiple small cracks in the resin and fibers of a composite structure. Typical crack sizes are in the 0.010 inch to over 0.200 inch range. Thermal damage may be attributable to various sources and, in aerospace applications, may be attributable to engine exhaust impingement, overheated components in a wheel well or other confined space, or fires involving an auxiliary power duct or other component. Regardless of its source, thermal damage degrades the matrix properties and the interface between matrix material and the embedded fibers, thereby decreasing the structural properties of the composite material and oftentimes requiring repair or at least more frequent monitoring.
Conventionally, laboratory-based methods have been employed to detect and determine the extent of thermal damage. Unfortunately, the laboratory-based methods cannot generally be performed in the field and oftentimes require disassembly or other rework of the composite structure. As such, non-destructive methods of detecting thermal damage have been developed, including infrared (IR) spectroscopy, laser pumped florescence and high frequency eddy current inspection. However, IR spectroscopy and laser pumped fluorescence are generally localized techniques capable of measuring thermal damage within one to three plies of the surface. For thicker structures, plies must generally be successively removed and then the remaining structure re-inspected to detect thermal damage deeper within a structure, thereby increasing the time and cost required for an inspection. High frequency eddy current inspection measures the change in resistance in the matrix material, such as that change in resistance attributable to overheating. However, high frequency eddy current inspection is also a near surface inspection method and generally cannot be utilized if the composite structure includes lightening strike protection. High frequency eddy current inspection is also disadvantageously sensitive to conductive structures in the immediate vicinity of the inspection area and to the geometry of the structure.
Ultrasonic PE has also been employed in an effort to detect thermal damage. However, it may be difficult to detect thermal damage until the thermal damage is sufficiently substantial so as to result in discrete delaminations. Accordingly, thermal damage may be difficult to detect and/or quantify via ultrasonic PE at earlier stages of degradation.
In some instances, the thermal damage is not visible and conventional nondestructive inspection techniques do not detect the thermal damage, particularly in instances in which the composite material must be inspected from a single side for at least the reasons described above in conjunction with porosity detection. Moreover, even in instances in which it is suspected that a composite structure has suffered thermal damage, such as a result of surface charring or discoloring, a portion of the composite structure may be removed and replaced. However, the removal and replacement may later prove to be completely unnecessary in instances in which the composite structure has, in fact, not been thermally damaged. Alternatively, the removal and replacement may later prove to be excessive in instances in which a larger portion of the composite structure is removed and replaced out of precaution than has been actually thermally damaged.